The interior to be thermally conditioned, most notably in vehicles used for passenger transport, e.g., buses, is relatively large, so that correspondingly dimensioned thermal conditioning units have to be used. In this case, it must be possible to cool the vehicle interior in the summer, while the vehicle interior has to be heated, for example, in the winter. The units intended for thermal conditioning shall be operable independently of the drive mode of such a vehicle, so that a necessary conditioning corresponding to the external thermal conditions can be carried out even in case of longer parking phases.
For cooling vehicle interiors, air conditioning systems operating with a refrigerant circuit are used, the condenser or condenser heat exchanger assembly of which discharges heat from the compressed refrigerant towards the environment, while in the evaporator or the evaporator—heat exchanger assembly, heat is drawn from the air flowing around said assembly in the evaporation process of the refrigerant, so that this air can be introduced, in a cooled state, into a vehicle interior. If such an air conditioning system with a refrigerant circuit shall be operated during the heating operation as well, then it is generally necessary to reverse the refrigerant circuit. This requires a complicated line routing with numerous valves. An alternative possibility for operating such an air conditioning system both during the cooling operation and during the heating operation is the provision of an intermediate heat transfer circuit. In this case as well, a complicated line routing with additional pumps and valves is necessary.